Such a detector unit, which is previously known from U.S. Pat. No. 5,308,987 (Wuest et al) comprises:
a solid material structure including a conversion medium in which incoming radiation causes the emission of electrons into at least one drift region, PA1 an anode and cathode for generating an electric field in said at least one drift region, and PA1 a detector sensitive in two dimensions adjacent to said solid material structure so as to detect electrons originating from said at least one drift region in response to said incoming radiation.
In the previously knowndetector unit the solid material structure is formed by a solid layer made of a conversion medium. The incoming radiation interacts with the conversion medium and generates electrons which are transported by means of the electric field into an adjacent gas chamber where a detector is located. Normally, secondary electrons are generated in the gas chamber, possibly under avalanche amplification. The detector is coupled to readout electronics.
Accordingly, a two-dimensional detection of the radiation can be accomplished. However, the efficiency of the prior detector unit will not be very high because the absorption probability of the incoming radiation is low, and only a small fraction of the electrons, which are generated in the vicinity of the surface of the conversion medium, will be emitted into the gas chamber.
Also, the low efficiency and a necessarily long absorption length for the radiation in the gas results in the positional resolution being relatively low.